Conventional methods of manufacturing semiconductor devices comprise the formation of field oxide regions for isolating active regions in the surface of a semiconductor substrate. The field oxide regions are unavoidably formed with an end portion, which borders the active region, which end portion resembles and, therefore, is commonly referred to as a "bird's beak." See, for example, Liu et al., U.S. Pat. No. 5,151,381.
Conventional methods of forming a field oxide region typically comprise depositing a first or pad-oxide layer on the semiconductor substrate, and depositing a nitride layer thereon. Patterning and etching are then conventionally conducted to form openings extending to and exposing the semiconductor substrate and side surfaces of the nitride and pad-oxide layers. The openings define areas on the surface of the semiconductor substrate in which field oxide regions are subsequently thermally formed, while the areas under the pad-oxide represent future active regions. A channel-stopper region or field implant is then formed in the semiconductor substrate, using the pad-oxide and nitride layers as a mask, under the region wherein the field oxide region is to be subsequently formed. Thus, the field implantation involves the ion implantation of an impurity having the same conductivity as the impurity in the semiconductor substrate. In N-MOS circuits, a P.sup.+ implantation of boron is typically employed.
There are known problems associated with channel-stopper regions. See, for example, Wolf, Silicon Processing for the VLSI Era--Volume II--Process Integration, Lattice Press, 1990, pp. 22-23, Sections 2.2.2.4 and 2.2.2.5., wherein it is recognized that during field oxidation, the channel-stop implant experiences segregation and oxidation enhanced diffusion. Consequently, relatively high doses are required to achieve an acceptable field threshold voltage, and the peak of the implant must be deep enough so that it is not absorbed by the growing field-oxide interface. If the channel-stop doping is too heavy, high source/drain-to-substrate capacitances result. In addition, the heavy channel-stop doping reduces the source/drain-to-substrate pn junction breakdown voltages. It is also disclosed that, as a result of lateral diffusion, the field implant impurity encroaches on the active regions, thereby increasing the threshold voltage for narrow channel widths. In addition, dislocations generated during the channel-stop implant penetrate nitride-edge-defined junctions causing increased emitter-base leakage in bipolar devices. Wolf reports that prior attempts to address such problems attendant upon field implants include the use of high pressure oxidation to grow the field oxide region, a geranium-boron co-implant, and a chlorine implant.
There exists, however, a need for an efficient simplified technique to overcome the problems associated with the formation of a field implant or channel-stopper region under the field oxide region, particularly a solution to the narrow channel effect, notably at high voltages, high junction peripheral capacitance and low junction breakdown voltages. This need is particularly acute for circuits that employ high voltages on or across the field isolation region.